Riki Kato is on a mission to find out how a collection of famous portraits depicting indigenous Ainu chieftains of present-day northern Japan ended up on the other side of the world, in eastern France.

"Why did these works equivalent to national treasures end up in France?," asks the 67-year-old former newspaper reporter, adding that he wanted Hokkaido authorities to launch an investigation into the mystery of the "Ishuretsuzo" series of paintings, which are over 140 years old.

Portraits from the "Ishuretsuzo" series depicting indigenous Ainu chieftains of present-day northern Japan. (Copyright the Museum of Fine Arts and Archeology of Besancon)(Kyodo)

The masterpieces were completed in 1790 by Hakyo Kakizaki, a painter and retainer of the Matsumae domain on Japan's northernmost main island and form a series depicting 12 chieftains who helped Japanese forces suppress Ainu uprisings during the Menashi-Kunashir rebellion in 1789.

In 1933, the series, minus one painting that had gone missing, surfaced in a warehouse at the Museum of Fine Arts and Archeology of Besancon, France. The paintings were proven to be authentic in the 1980s through analyses.

There are numerous theories as to how the paintings got there from Japan, Kato says, including that they were brought back by a French soldier who fought in the 1868-1869 Battle of Hakodate, or by Kakizaki's eldest son.

The most likely candidate, Kato says, is Eugene-Emmanuel Mermet-Cachon, a French missionary who lived in Hakodate.

Although there is no conclusive evidence, it is believed that the paintings were a gift to Mermet-Cachon from the Hakodate Magistrate's Office, and they entrusted to a missionary from Besancon, who passed them on to his brother, a curator at the museum.

Kato first learned of the paintings in 1984, when he was a reporter for the Hokkaido Shimbun newspaper, and the art work featured on its front page. In 1987, Kato had the opportunity to see the paintings in person, even though they were not on public display, and was struck by their fine detail and vivid colors.

After becoming the paper's Paris correspondent in 1994, Kato eagerly interviewed art curators and descendants of military personnel, hoping to find clues, but was unable to solve the mystery.

Even after retiring in 2021, Kato remains unfulfilled and continues to interview curators while also giving lectures at French language schools.

In April 2022, Kato published a book that summarized his research, which he did at his own expense.

Meanwhile, the paintings are still in storage at the museum. Kato hopes that they can one day be shown to the public in Japan, especially in Hokkaido.

"I hope that government agencies and researchers will work together in efforts to reach out to France, so we can deepen our understanding of the Ainu people and rekindle momentum for solving the mystery," he said.

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